Writes the substring specified by string, start, and
end to output-port and returns an unspecified value.
Equivalent to writing the characters of the substring, one by one, to
output-port, but is implemented very efficiently.

— operation on output port: fresh-line output-port

Most output ports are able to tell whether or not they are at the
beginning of a line of output. If output-port is such a port,
end-of-line is written to the port only if the port is not already at
the beginning of a line. If output-port is not such a port, an
end-of-line is unconditionally written to the port. Returns an
unspecified value.

— operation on output port: flush-output output-port

If output-port is buffered, this causes its buffer to be written
out. Otherwise it has no effect. Returns an unspecified value.

— operation on output port: discretionary-flush-output output-port

Normally, this operation does nothing. However, ports that support
discretionary output flushing implement this operation identically to flush-output.

Returns the number of unwritten characters that are stored in
output-port's buffer. This will always be less than or equal to
the buffer's size.

— operation on output port: output-buffer-size output-port

Returns the maximum number of characters that output-port's buffer
can hold.

— operation on output port: set-output-buffer-size output-port size

Resizes output-port's buffer so that it can hold at most size
characters. Characters in the buffer are discarded. Size must be
an exact non-negative integer.

— operation on output port: x-size output-port

Returns an exact positive integer that is the width of output-port
in characters. If output-port has no natural width, e.g. if it is
a file port, #f is returned.

— operation on output port: y-size output-port

Returns an exact positive integer that is the height of
output-port in characters. If output-port has no natural
height, e.g. if it is a file port, #f is returned.

— procedure: output-port/x-size output-port

This procedure invokes the custom operation whose name is the symbol
x-size, if it exists. If the x-size operation is both
defined and returns a value other than #f, that value is returned
as the result of this procedure. Otherwise, output-port/x-size
returns a default value (currently 80).

output-port/x-size is useful for programs that tailor their
output to the width of the display (a fairly common practice). If the
output device is not a display, such programs normally want some
reasonable default width to work with, and this procedure provides
exactly that.

— procedure: output-port/y-size output-port

This procedure invokes the custom operation whose name is the symbol
y-size, if it exists. If the y-size operation is defined,
the value it returns is returned as the result of this procedure;
otherwise, #f is returned.